A big red dot at cosmic noon

Federica Loiacono, Roberto Gilli, Marco Mignoli, Giovanni Mazzolari, Roberto Decarli, Marcella Brusa, Francesco Calura, Marco Chiaberge, Andrea Comastri, Quirino D'Amato, Kazushi Iwasawa, Ignas Juodžbalis, Giorgio Lanzuisi, Roberto Maiolino, Stefano Marchesi, Colin Norman, Alessandro Peca, Isabella Prandoni, Matteo Sapori, Matilde Signorini, Paolo Tozzi, Eros Vanzella, Cristian Vignali, Fabio Vito, Gianni Zamorani

First listed 2025-06-13 | Last updated 2025-09-09

Abstract

We report the discovery of a little red dot (LRD), dubbed BiRD ('big red dot'), at $z=2.33$ in the field around the $z=6.3$ quasar SDSSJ1030+0524. Using NIRCam images, we identified it as a bright outlier in the $F200W-F356W$ color vs $F356W$ magnitude diagram of point sources in the field. The NIRCam/WFSS spectrum reveals the emission from HeI$λ10830$ and PaG line, both showing a narrow and a broad ($FWHM\gtrsim 2000\ \rm kms^{-1}$) component. The HeI line is affected by an absorption feature, tracing dense gas with HeI column density in the $2^3S$ level $N\sim 0.5-1.2\times 10^{14}\rm cm^{-2}$, depending on the location of the absorber, which is outflowing at the speed of $Δv \sim -830\ \rm kms^{-1}$. As observed in the majority of LRDs, BiRD does not show X-ray or radio emission. The BH mass and the bolometric luminosity, both inferred from the PaG broad component, amount to $M_{\rm BH}\sim 10^8\rm M_{\odot}$ and $L_{\rm bol}\sim 2.9\times 10^{45}\rm ergs^{-1}$, respectively. Intriguingly, BiRD presents strict analogies with other two LRDs spectroscopically confirmed at cosmic noon, GN-28074 ("Rosetta Stone") at $z=2.26$ and RUBIES-BLAGN-1 at $z=3.1$. The blueshifted HeI absorption detected in all three sources suggests that gas outflows may be common in LRDs. We derive a first estimate of the space density of LRDs at $z<3$ based on JWST data, as a function of $L_{\rm bol}$ and BH mass. The space density is only a factor of $\sim 2-3$ lower than that of UV-selected quasars with comparable $L_{\rm bol}$ and $z$, meaning that the contribution of LRDs to the broader AGN population is also relevant at cosmic noon. A similar trend is also observed in terms of BH masses. If, as suggested by recent theories, LRDs probe the very first and rapid growth of black hole seeds, our finding may suggest that the formation of black hole seeds remains efficient at least up to cosmic noon.

Short digest

JWST reveals BiRD, a luminous little red dot at z=2.33 in the J1030 field, flagged as a bright outlier in the F200W−F356W vs F356W diagram. NIRCam/WFSS detects He I λ10830 and Paγ with both narrow and broad components (FWHM ≳ 2000 km s−1), plus a blueshifted He I absorption indicating dense outflowing gas with N(He I*, 2^3S) ≈ 0.5–1.2×10^14 cm−2 at Δv ≈ −830 km s−1; the source lacks X-ray and radio emission. The Paγ broad component implies MBH ≈ 10^8 M⊙ and Lbol ≈ 2.9×10^45 erg s−1, echoing GN-28074 and RUBIES-BLAGN-1 where similar He I absorption is seen. A first JWST-based census places the LRD space density at z<3 within a factor ∼2–3 of UV-selected quasars at comparable Lbol and MBH, arguing that LRDs contribute meaningfully to AGN demographics at cosmic noon and may trace rapid seed growth.

Key figures to inspect

• Figure 1: Verify BiRD’s placement above the stellar locus in F200W−F356W versus F356W, its redshift track, and its proximity to the Rosetta Stone and RUBIES-BLAGN-1 points; this sets the color-based selection used for the space-density estimate.
• Figure 2: Inspect the optical–IR cutouts to confirm the compact, point-like morphology and the increasingly red continuum across NIRCam bands that underpin the LRD classification.
• Figure 3: Use the 2D/1D WFSS spectra to confirm z from He I λ10830 and Paγ, note the additional O I emission, and check the red-end contamination flagged in the panel.
• Figure 4: Read off the multi-component fits—narrow+broad Paγ (for MBH, Lbol) and He I emission plus the blueshifted He I absorption—to assess the ≳2000 km s−1 widths and the absorption kinematics and depth.